Ultrasound imaging provides useful information about the interior characteristics of an object or subject under examination. Conventional B-mode ultrasound imaging is performed by actuating a set of transducer elements to form an ultrasound beam having a fixed transmit focal point and sweeping the ultrasound beam through an examination area while transmitting pulses and detecting echoes. The echoes are delayed and summed to form B-mode scan lines, which are processed to generate images, which can be displayed via a monitor.
Harmonic imaging is similar to B-mode imaging except that one or more harmonic components (instead of the fundamental component) of the echoes are processed to generate the image. With harmonic imaging, the second harmonic component, for example, will have a frequency that is two times the frequency of the fundamental component, which results in a higher resolution image, relative to B-mode imaging. Unfortunately, harmonic imaging approaches have not been well-suited for deep penetration harmonic imaging (e.g., ten centimeters or greater) with the harmonics tending to dampen out between five to ten centimeters.
As such, harmonic imaging generally is not used for deep scanning. A solution has been to use harmonic imaging for shorter depths and then switch to from harmonic imaging to B-mode imaging at longer depths, even though the transducer is being operated in harmonic imaging mode. However, when imaging objects such as kidney stones, which generally are located at longer depths, artifact such as ghosting or mirroring may be introduced into the images. In view of the above, there is an unresolved need for other approaches to harmonic imaging.